In CMOS process, a MOSFET is generally used as one of the components of an RF power detector. It requires a sufficiently wide bandwidth enough to support signal detection in an RF band, and therefore, a complex structure such as a received signal strength indication (RSSI) is difficult to be applied to the power detector. Even if possible, power consumption is increased.
FIGS. 1a and 1b are views for describing the attribute of a MOSFET used in a prior art power detector. More specifically, FIG. 1a shows a prior art NMOS RF power detector 10a which utilizes a voltage-current conversion characteristic of an active area of NMOS. FIG. 1b shows a prior art NMOS RF power detector 10b which utilizes a voltage-current conversion characteristic of a linear area of NMOS (see [1] Yijun Zhou et al., “A Low-Power Ultra-Wideband CMOS True RMS Power Detector,” in IEEE Transactions on Microwave Theory and Techniques, Vol. 56, No. 5, May 2008, pp. 1052-1058; and [2] Kenneth A. Townsend, et al., “A Wideband Power Detection System Optimized for the UWB Spectrum,” in IEEE JSSC., Vol. 44, No. 2, February 2009, pp. 371-381, the entireties of which are hereby incorporated herein by reference).
The output currents iout of the power detectors 10a and 10b shown in FIGS. 1a and 1b can be expressed in the following equation.
                                          i            out                    ⁡                      (            t            )                          ∝                              k            2                    ⁢                      V            in            2                                              equation        ⁢                                  ⁢                  (          1          )                    
In FIGS. 1a and 1b, since RL is connected in parallel to CL, R∥C load impedance shown in FIGS. 1a and 1b can be expressed by the formula
      Z    L    -                    R        L                    1        +                  jω          ⁢                                          ⁢                      R            L                    ⁢                      C            L                                .  Here, “ω” represents an angular frequency. The output voltage Vout of the power detector is ioutZL and can be expressed by
          ⁢            V      out        =                  k        2            ⁢                                                  V              in              2                        ⁢                          R              L                        ⁢                                1            +                          jω              ⁢                                                          ⁢                              R                L                            ⁢                              C                L                                                    .            Here, 1/(1+jωRLCL) represents a low pass filter of which the cut-off frequency is 1/RLCL. When the 1/RLCL is sufficiently lower than the frequency of Vin, the 1/(1+jωRLCL) can function as a kind of an integrator.
“k” represents a process coefficient of the MOSFET, and therefore, has a large variation to process and temperature changes. Accordingly, the prior art power detector has a large process and large temperature change and is difficult to be applied to application products requiring accurate power detection.